Coin Cleaning: Mechanical, Chemical & Electrochemical Methods

Coin collection is a hobby for people, they collect them from one generation and pass them to the next generation. Therefore, these collected coins are valuable to them. When coins are stored for a long time, they oxidized and show dark colors and patina (green or brown film of material) with an extra mass of corrosion. Cleaning and restoring such kinds of coins is a challenging task.

Before cleaning the coins, they should be soaked for two days in the solutions, such as warm soapy water, olive oil, mineral oil, etc. This helps to loosen the dirt and other impurities from the surface of the coin easily. Using warm soapy water is an easy process to remove dirt and mud from the coin.

Olive oils are an effective method to remove extra dirt but soaking for a longer time in the olive oil leads to a chemical reaction between the coin and olive oil. This is due to the presence of fatty acids and sulfur compounds in olive oil. However, soaking in mineral oils is a safer method than olive oil because it does not have any fatty acids or other attacking compounds.

Coin cleaning

These methods are used to clean metal coins:

  • Mechanical method
  • Chemical method
  • Electrochemical and electrolytic method

Mechanical Method

In the mechanical method, different tools and techniques are used to remove impurities from coins. The mechanical method is used with a combination of chemical and electrical methods.

These are the tools and techniques used in the mechanical method to clean coins:

  • Brushes
  • Toothpick or dental pick
  • Masking Tape
  • Hard eraser
  • Tumblers
  • Carpenter’s glue method
  • Wet cleaning

Cleaning with brushes

Cleaning coins with a brush is very simple and can be done at home. Toothpaste can be used with a brush to remove impurities. For example, when a silver coin is washed with soapy water and then cleaned with a brush and toothpaste, it gives better results.

When the dirt and impurities do not remove from the coins then place them in the olive oil. They should be soaked for 1 to 2 days in olive oil. Then remove coins from the olive oil and brush them again.

Cleaning with a toothpick or a dental pick

Dirty coins can be cleaned by using a needle-like instrument such as a toothpick or dental pick.

Sometimes, dirt goes into that area where it remains for a long time. To remove such kind of dirt or mud, a toothpick or dental pick is used. A dental pick is made up of steel or other metal, so it should be used carefully so that it does not make scratch the surface of the coin.

Masking Tape

During cleaning, the surface of coins can be protected by using mask tape. When a toothpick is used to clean the coin, they leave small scratches on the surface of the coin. This is better to use masking tape on the surface of the coin than use a toothpick or dental pick for cleaning.

Hard eraser

Hard erasers are also used to clean dirt and other organic impurities from coins. These erasers are used when a small amount of hard dirt appears on the surface of coins.

Tumblers

Tumblers are basically used to polish rocks. They can also be used in the cleaning of coins. To clean a coin with a single brush take time. To save time and get better results, tumblers are used.

Carpenter’s glue method

Carpenter’s glue is an aliphatic resin emulsion known as yellow glue. It is the best choice to remove dirt encrustation in spots or edges. First, the glue is heated then apply to the surface of the coin. When the glue is cool down remove it from the coin which also removes the dirt.

Wet cleaning

This is a very simple method in which a coin is first soaked in the soapy solution and then it is rubbed with the fingers. In this way, by rubbing the wet coin between the fingers, remove small particles of dirt. It is also enhanced by adding salt to the wet coin and then start rubbing.

Chemical Method

Cleaning coins with chemicals is the best and easy method. Chemicals react with the dirt (organic compounds) and other substances present on the surface of coins. For this purpose, it is necessary to merge coins completely in the solution of chemicals. Chemicals used for this process are dilute.

When using concentrated chemicals to remove dirt from the surface of the coin, they can harm the coin and dissolve them in the solution. Secondly, they change the color of the coin and turn them into yellowish black or dark colors. However, to maintain the smoothness of the coins, a dilute solution of chemicals or less reactive chemicals is used.

To get better results, it is good to leave the coins in the solution for 24 hours. In this way, it will completely dissolve the dirt and corrosion.

Steps to clean coins with chemicals

  1. Add chemicals with water in the glass or plastic chemical. When using a strong chemical like acid, it is necessary to use a glass container. If acid or base is used as a chemical then it should be strong enough to dissolve dirt, not the whole coin. It is because sometimes strong chemicals are used to dissolve coins.
  2. Leave the coin in the solution for minimum of 24 hours.
  3. After that, remove the coins from the solution and check whether it is cleaned or not. If not cleaned properly, leave it for further 24 hours in the solution.
  4. After removing dirt and corrosion from the surface of the coin, now it is time to rinse the coin with warm water.
  5. Finally, dry the coin with soft material like a towel, etc.

Chemicals for Cleaning coins

These are the chemicals that are used for cleaning the coins:

Acids

When using acids for coin cleaning purposes. It should be dilute.

  • Dilute sulfuric acid (H2SO4)
  • Dilute nitric acid (HNO3)
  • Dilute hydrochloric acid (HCl)
  • Hydrogen peroxide
  • Vinegar
  • Lemon juice

Bases

Strong bases are also not good to use for coin cleaning. Bases are mostly used for fully encrusted coins.

  • Sodium hydroxide (NaOH)
  • Potassium hydroxide (KOH)
  • Baking soda (NaHCO3)
  • Ammonium hydroxide (NH4OH)

Ion substances

Ions are also involved to remove impurities from the surface of coins.

  • Sodium chloride (NaCl)
  • Potassium chloride (KCl)
  • Sulfur derivatives

Coins Immersion bath

An immersion bath is a type of process which is used to remove patina and impurities from the surface of the water. In this process, immerse an uncleaned coin in the acid, base, or salt solution depending on the type of coin.

These are the following type of immersion baths:

Lye bath

A Lye bath can be prepared by adding solid lye and water in a 1:3 ratio. Coins are soaked in the lye solution for several minutes. This depends on the type of metal coin. For example, some coin takes less time to be cleaned and other takes a week or month.

It is the best chemical method to clean fully encrusted coins. Secondly, it saves the coin’s upper layer and gives better results at the end. Lye is dangerous for humans thatswhy it should be used carefully.

Hydrogen peroxide bath

Hydrogen peroxide is an effective bath for uncleaned coins. When coins are soaked in the solution of hydrogen peroxide, the production of bubbles takes place which is the indication of dissolving dirt and other compounds.

Ammonia solution bath

A metal coin can be merged in the 10% solution of ammonia (NH4OH) to remove the metal oxides and crust, etc. This process can be carried out in a closed container. The changing color of the solution from clear to dark blue or violet is an indication of the reaction between coin and ammonium hydroxide.

Vinegar and salt solution bath

Nickel metal coins and pennies are cleaned by using vinegar and salt solution. This is a very effective and inexpensive chemical technique. Normally, a glass jar is used for this purpose add vinegar to about half a jar and then add coins, and remain them in the jar for 5 minutes. Then add table salt to the solution and stir with a wooden stick.

Stirring of the solution continues until the coins are clean. After the vinegar and salt bath, coins should be rinsed with distilled water. Secondly, weak acid or dilute acid can be used to neutralize the reaction on the coins after vinegar and salt-action.

Olive oil and lemon bath

This process is used to remove the oxide layers or minor impurities from the surface of silver and copper coins. The solution can be prepared by adding one teaspoon of lemon juice to the 1.5 cups of olive oil. The mixture should be mixed well and then place coins in it.

Now add water to fill the bottom of the container or pot in which the solution and coins are carried. Then place it on the flame for a time, until the temperature is raised to 70 to 80 degrees. In this way, impurities from the surface of silver and copper coins can be removed easily.

Electrochemical and electrolytic Methods

Metal coins can be cleaned by using electrical methods, such as electrochemical reduction and electrolysis. In the electrochemical reduction, coins are cleaned by using metal reduction. However, in electrolysis, the coin’s impurities are removed by the mechanical action of hydrogen gas.

Electrochemical reduction

The layer of oxides on the surface of coins can be removed by reduction which decomposes the corrosion and separated them into their atoms like oxygen, chlorine, sulfur, etc. Normally, these atoms are responsible for oxidation or corrosion. In the electrochemical reduction, the current produced is known as the galvanic current between the two electrodes, the anode, and cathode.

Electrolysis

It is the chemical decomposition of metals by direct electric current. When a coin is hard to clean and all other techniques failed to remove deposits from the coin then electrolysis is used. When the electrolysis starts, it breakdown the water into oxygen and hydrogen. This result is the evolution of hydrogen bubbles in the solution. In this way, hydrogen works mechanically on the surface of the coin and breaks the encrustation and patina.

How to clean coinage metals?

Copper, silver, and gold are the metals that are used for making coins. They are also called coinage metals.

These are some techniques to clean coinage metals:

  • They can be cleaned by using wet cleaning and cleaning with brushed.
  • Coinage metals can also be cleaned with Soft erasers, toothpicks, etc.
  • Immersion of coinage metals in the solution of hydrogen peroxide gives good results.
  • 10% solution of ammonium hydroxide cleans the copper, zinc, and gold coins.
  • Usage of dilute acids removes the impurities from the surface of coinage metals.
  • Spirit and methyl alcohol is also good cleaner.
  • Zinc coins can be easily cleaned by using vitrolin copper soap.

Concepts Berg

What liquid cleans coins the best?

These are the liquid that can clean coins very effectively.

  • Vinegar
  • Soapy water
  • Saltwater
  • Lemon juice

Can vinegar damage coins?

Vinegar is the best source to clean metal coins. Metal coins are soaked in the vinegar and water solution. It should be kept for a few hours. When coins are soaked for a long time in vinegar, it can harm the surface of the coin.

What is the safest way to clean a coin?

Weak chemicals, such as vinegar, salt water, and olive oil are the best and safest methods for cleaning metal coins.

How to clean a coin that turned green?

The brown-green color on the surface of the coin is called patina. This can be cleaned by using different methods, such as chemical, mechanical and electrical methods.

What are the best ways to clean rusty coins?

Rusty coins can be easily cleaned by using electrolysis which gives a better result. The alternative way is washing with weak acid and bases.

How to effectively clean a penny and other old coins?

Using olive oil is an effective way to remove the impurities from the surface of the penny and old coins.

How to clean coins with baking soda?

Baking soda can be used to clean coins by mixing them with toothpaste and salt water.

How to clean coins with hydrogen peroxide?

To clean metal coins with hydrogen peroxide, give a hydrogen peroxide bath for several minutes with water.

Reference Books

  • The Art of Cleaning Ancient Coins by Kevin R Sandes

Reference links

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